1. Field of the Invention
The invention relates generally to sound reproducing systems and in particular to such systems which include motion sensing means in a feedback path.
2. Description of the Prior Art
There are numerous solutions of the problem of motional feedback in sound reproducing systems for improving frequency and transient responses as well as reducing distortions and loudspeaker dimensions. Common idea of existing solutions consists in transforming the motion of the loudspeaker sound generating member, most frequently a cone, into an electrical signal and applying this signal degeneratively into the feedback loop, forcing the loudspeaker to respond linearly to the input acoustic signal.
Such prior art sound reproducing systems, utilizing an auxiliary coil or a light beam and a photocell as a feedback signal source, are cited in U.S. Pat. No. 2,194,175, issued to Wilhelm.
Conrad, in his U.S. Pat. No. 2,860,183, describes sound reproducing systems deriving the feedback signal from an auxiliary winding on the voice coil bobbin, a piezoelectric transducer or a movable magnetic diaphragm.
De Koning in U.S. Pat. No. 3,334,184, issued to him, describes motional feedback by means of a piezoelectric transducer of acceleration.
U.S. Pat. No. 3,057,961, issued to Turner, discloses self-correcting sound reproducing apparatus with capacitive, mechano-electrical feedback transducer.
Method of transformation of loudspeaker cone motion into an electrical signal by means of a magnetoresistive transducer has been disclosed by Cuno in his U.S. Pat. No. 4,176,305.
Further, Meyers in his U.S. Patents No. 3,798,374 and No. 4,256,923 discloses motional feedback by means of a sensing coil interacting with a magnetic field of a permanent magnet or interacting with a stray field of a loudspeaker magnet.
Among disadvantages of the prior art systems are: necessity of adding substantial mass to the moving system of the loudspeaker, electrical interference from the electromagnetic system of the associated loudspeaker and, what is most important, they are sensitive to first or even second derivative of the cone displacement with respect to time rather than to the cone displacement as such. In other words, the prior art motional transducers detect the speed or even the acceleration of the sound generating member, what renders them frequency dependent, necessitates application of complex equalizer circuits and at very low acoustic frequencies makes them virtually useless due to substantial decrease of the feedback signal.